Generally, the preparation of the paper pulp comprises separating the elements forming wood, and more particularly isolating cellulose fibers from the other components (mainly wood lignin).
It may be an alkaline wood processing method, for example, by cooking in the presence of a solution called “white liquor”. White liquor is generally formed of water, of soda, and possibly of a source of sulfur such as sodium sulfide. The alkaline processing of wood is generally performed at a temperature in the range from 150 to 180° C., and at a pressure in the range from 7 to 9 bars. This is in particular true for the preparation of Kraft-type paper pulp.
At the end of the processing, the cellulose-rich fibers form the paper pulp, while the cooking residues form an aqueous phase called “black liquor”. Black liquor contains a large part of the wood lignin, possibly in a degraded state, part of the hemicelluloses, but also of the degraded cellulose and of the derivatives of the cooking reactants present in the white liquor (sodium carbonate . . . ).
Typically, the black liquor is recycled by being concentrated and then burnt in a Tomlinson-type boiler. The combustion of the black liquor generates heat and vapor which may be used in the pulping process.
In an alkaline method, the black liquor combustion residues are generally treated to recover the nitrogen as well as the sulfur originating from the reactants initially present in the white liquor.
This type of alkaline method has many disadvantages, among which:
the cost of installing and maintaining the boiler allowing the combustion of the black liquor;
the low energy efficiency of the black liquor combustion (less than 40%); and
the impossibility of unclogging the black liquor boiler to increase the pulping yield.
It is possible to extract the hemicelluloses prior to the cooking of the wood. Such a pre-processing is particularly advantageous, given that hemicelluloses may be a source of sugars, or oligomers, or of polymers usable in the manufacturing of bio-products (surface-active agents . . . ), of bio-fuel (ethanol . . . ), or of biomaterials.
On the other hand, the prior extraction of hemicelluloses makes the pulping easier, since it enables to get rid of the presence of sulfur.
Other black liquor usage methods have been studied, for example, to chemically use the lignin of the black liquor. Document U.S. Pat. No. 2,399,607 describes the forming of vanillin by wet oxidation (WO) of the black liquor.
Document EP 0 251 533 describes the wet oxidation of a portion only of the black liquor prior to the steps of evaporation and combustion in the boiler. In this method, the organic material of the treated black liquor portion is destroyed by more than 90%. The oxidized black liquor is then mixed with the remaining black liquor to be treated according to the conventional method (concentration+combustion). The organic load of the black liquor is thus decreased, which enables to unclog the black liquor combustion boiler.
However, such solutions are not fully satisfactory.
The present invention enables to combine the utilization of organic compounds contained in the black liquor and the unclogging of the black liquor combustion boiler and this, without impacting the regeneration of wood cooking reactants, that is, soda in the alkaline method.